Method for simultaneous tightening of two or more screw joints

ABSTRACT

A method for simultaneous tightening of two or more screw joints to a desired pretension condition by means of a nut running tool having two or more output spindles each having an adjustable output torque, wherein each one of the output spindles is individually operated according to at least one primary tightening parameter from among torque, angular position, time, clampload, and screw joint elongation. An instantaneous value of the at least one primary tightening parameter is continuously detected for each one of the output spindles, and the instantaneous tightening parameter value for each one of said output spindles is continuously compared with a corresponding instantaneous tightening parameter value of each other one of the output spindles. A torque application speed is then continuously adjusted by continuously adjusting a limit of the output torque of one of the output spindles on which the instantaneous tightening parameter value exceeds a predetermined boundary in relation to any other one of the spindles.

The invention relates to a method for simultaneous tightening of two ormore screw joints by means of a nut running tool having two or moreoutput spindles.

BACKGROUND OF THE INVENTION

A problem concerned with previous tightening methods for pretensioningtwo or more screw joints at the same time is to obtain a balanced andeven clamping force build-up between the parts being joined by the screwjoints. One such method is described in Japanese Patent Application No.:JP 900335746 (Publication No.: JP 4201022). According to this knowmethod, all joints are run down in a first step to a seated condition.Thereafter, the pretensioning phase is started for all joints, andduring an initial stage, the individual torque growth characteristicsfor the joints are determined followed by an adjustment of the rotationspeed of each spindle so as to aim at a simultaneous arrival of thescrew joints to the desired torque level.

This known method for tightening process control is disadvantageous anthat it is too slow, not only due to the tightening process beingcarried out in two steps, but also due to the torque growth adaptationby speed adjustment being too slow. The control of the process is alsoinsufficient due to the speed adjustment being carried out only once.This may be sufficient to obtain some compensation at ideal jointshaving linear torque growth characteristics, but will result inunacceptable differences in tightening time for reaching a commonpredetermined torque or clamping force level at joints having differentnonlinear torque rates, i.e. different torque/angle characteristics.This in turn would result in an undesired uneven claping forceapplication on critical machine parts like shaft bearing caps andcylinder heads of internal combustion engines.

Another drawback concerned with this and other known methods forsimuitaneous tightening of two screw joints is the risk for damage tothe equipment and injury to the operator in case one of the two outputspindles loosens its grip on the screw head or nut being tightened,because if the reaction support for one of the spindles is lost theentire nutrunner is exposed to the reaction torque from the otherspindle. This is a safety risk for the operator who has to counteractthe reaction torque by muscle force.

SUMMARY OF THE INVENTION

An object of the invention is to create a method for simultaneoustightening of two or more screw joints by means of a nut runing toolhaving two or more output spindles, whereby a balanced and even clampingof the joint parts is obtained. According to one aspect of the inventionthis is accomplished by a tightening process wherein all spindles areoperated according to a predetermined tightening strategy with respectto at least one primary tightening parameter from among rotation speed,output torque, angle, time, clampload and screw joint elongation. Theinstantaneous value of such parameter on each one of the spindles iscontinuously detected and compared with the instantaneous parametervalue on the other spindles, whereupon the rotation speed iscontinuously adjusted on that one of the spindles on which the highest(or alternatively the lowest) instantaneous value is detected sio as tomaintain the continued torque load on that spindle within apredetermined boundary in relation to that one of the other spindlesmomentarily having the highest (or alternatively the lowest) torqueload.

According to an alternative aspect of the invention, the tighteningprocess is carried out under detection of the instantaneous value of aprimary tightening parameter on each spindle, and adjusting continuouslythe rotation speed of each spindle but one spindle to accomplish atightening parameter value on the individual spindle that is equal tothat of that one spindle, thereby accomplishing a fast and universallyuseful tightening method.

A further object of the invention is to form a tightening method for atwo-spindle nutruuner by which the safety risk related to a lost screwjoint grip is eliminated.

The invention is below described in further detail with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram illustrating tightening of two screw jointshaving an ideal linear torque rate.

FIG. 2 shows a diagram illustrating tightening according to a prior artmethod of two screw joints having different nonlinear torque rates.

FIG. 3 shows a diagram illustrating tightening by the method accordingto the invention of two screw joints having different nonlinear torquerates.

FIG. 4 shows a two-spindle power nutrunner for carrying out the methodaccording to the invention.

DETAILED DESCRIPTION

As illustrated in FIG. 1, tightening of for instance two screw jointswhich have ideal linear torque rates may be carried out at the sameconstant rotation speed and will reach the desired final torque orpretension level at the same time.

In the diagram in FIG. 1, the torque/angle curves of the two screwjoints are illustrated in straight solid lines, whereas the clampingforce characteristics as a function of time are illustrated in straightdash lines. It is also illustrated that the tightening of both joints iscarried out at the same constant rotation speed n₁.

The linear and equal clamping force characteristics of the twotightening processes illustrated in FIG. 1 are ideal and serve as amodel for the clamping force characteristic which is desireable toobtain at different types of screw joints with different non-lineartorque rates. Such identical and linear clamping force characteristicsfor two or more screw joints tightened simultaneously guarantee abalanced and even clamping force application on the parts being joined.

In the diagram shown in FIG. 2, there are illustrated in solid lines thetorque rate M/α curves of two different joints A and B. From the diagramit is apparent that the torque rates M/α of the two joints arecompletely different. In joint A, the torque/angle curve (M/α)_(A) has alow derivative during the first part of the tightening process, but hasa successively increasing derivative at increasing angle of rotation α.

As illustrated in dash line, the clamping force growth as a function oftime (P/t)_(A) will have a similar characteristic during the tighteningprocess if the rotation speed n_(A) of the nutrunrer spindle isconstant.

The same reasoning is applicable on screw joint B which has a torquerate (M/α)_(B) of a characteristic opposite to that of joint A. Thus,the derivative of the torqe/angle curve (M/α)_(B) of screw joint B ishgh at the start but decreases successively at increasing rotation angleα. If tightening joint B at the same constant rotation speed as by jointA, n_(A) =n_(B) the clamping force growth as a function of time(P/t)_(B) will have a characteristic similar to that of the torque rate(M/α)_(B).

If the two screw joints A and B were tightened simiultaneously at aconstant speed n_(A) =n_(B), there would be obtained a substantialdifference in clamping force ΔP.sub.Δt after a certain time intervalΔ_(t). See FIG. 2. This would result in an unbalanced and unevenclamping of the machine parts to be joined.

In contrast to the tightening processes illustrated in FIG. 2,tightening of the two screw joints A and B by the method according tothe invention means that the clamping force growth characteristics P/tas well as the torque growth characteristics M/t as a function of timeare equal. See FIG. 3. This also means that the instantaneous clampingforces P are equal for the joints. This is illustrated by the clampingforce comparison at a point Δ_(t) after the start of the two tighteningprocess, where P.sub.Δt is equal for the two joints.

This favorable clamping force characteristic for the two different screwjoints A and B is accomplished by adapting continuously the rotationspeed of the individual nutrunner spindles in relation to theinstantaneous torque load as illustrated by the non-linear rotationspeed curves n_(A) and n_(B).

In simple cases where the torque load increases continuously atincreasing angle, it suffices to adapt speed. In complex cases where thespeed might fall down to zero, the output torque as well has to becontrolled.

The method according to the invention means that in practice therotation speed and torque of each one nutrunner spindle is continuouslyand individually adjusted so as to adapt the instantaneous torque loadof each one spindle to remain within a predetermined torque span inrelation to the torque of a master spindle. The master spindle can bepreselected or preferably dynamically allocated. The master spindle isoperated in accordance with a predetermined rotation speed pattern so asto obtain a torque or clamping force characteristic expectable at theactual type of joint.

One way to apply the method with dynamic master allocation is that afirst priority for every spindle is to operate according to a programmedtightening strategy regarding speed, angle, torque, time etc.

At the same time there is monitored the progress of all the otherspindles and if the spindle in question runs ahead torquewise more thana certain amount in relation to anyone of the other spindles (dynmicmaster spindle) there will be an automatic reduction in speed of thespindle in question, or in stationary conditions: a direct output torquereduction to maintain the torque within the predefined boundary. Thereduction of torque, however, will always be less than 100% of thedifference to the reference spindle, otherwise a chasing effect couldoccur stopping all spindles.

This results in a balanced and even clamping of the parts to be joined.

The method according to the invention also comprises a feed back featureby which the master spindle and all other the spindles are shut off ifthe torque load on anyone of the spindles differs by more than a certainamount from the instantaneous torque load on anyone else of thespindles.

This method is also most suitable for avoiding the safety risk attwo-spindle nutruners, because if one of the two spindles looses itsgrip on the screw head or nut being tightened, the reaction torque fromthe other spindle would cause a rotation of the entire nutrunner whichwould be hazardeous to the operator. By the method according to theinvention, this is avoided by a prompt shut-off of both spindles due toa certain detected difference in the instantaneous torque load.

In FIG. 4, there is shown a two-spindle nutrunner suitable for operationin accordance with the above described method The nutrunner comprisestwo output spindles 10, 11 each rotated by an electric motor unit 12 and13, respectively. The motor units 12, 13 are rigidly secured to eachother at a predetermined relative distance by means of a cross piece 14.The distance between the spindles 10, 11 is determined by the distancebetween the screw joints to be tightened. In FIG. 4, there isillustrated two parts A and B being clamped together by two screws 16,17 the relative distance of which deterines the distance between theoutput spindles 10, 11 of the nutrunner.

In practice, the cross piece 14 is designed so as to enable adjustmentof the distance between the output spindles 10, 11 to adapt thenutrunner to different screw joint configurations. This is not, however,a part of the invention and is not described in further detail.

For manual operation of the nutrunner there are provided two laterallyextending handles 18, 19. These are secured to two brackets 21, 22rigidly clamped to the lower ends of the motor units 12, 13. The handlebrackets 21, 22 also form supports for suspension yokes 23, 24 by whichthe nutrunner is connectable to an overhead balancer via suitable cables(not shown).

The motor units 12, 13 are connected to a programmable operation controlunit 26. The latter comprises power supply modules for each of theelectric motor units 12,13. The control unit 26 is also connected to oneof the handles 19 which comprises a lever 27 for operating an on/offswitch, and light diods 28 for indicating a completed acceptedtightening process or, alternatively, a faulty tightening process.

The control unit 26 is programmed to accomplish a tightening process bya first one of the spindles in accordance with a desired predeterminedtorque load characteristic as a function of time. The rotation speed ofthe other spindle is continuously adjusted during tightening so as toadapt the torque load characteristic to that of the first spindle.Thereby, the first spindle acts as a master.

Should one of the two spindles 10, 11 loosen its grip or perhaps neverbe properly connected to the respective screw head, the reaction torquefrom the other spindle would cause a rotation of the entire nutrunner.The operator would be exposed to that reaction torque via the handles18, 19 and would have to counteract that reaction torque by muscle forceonly. In many cases that reaction torque would be too high and start toosudden for the operator to manager. Injury to the operator and damage tothe equipment might be the result.

It should be noted that the method of the present invention can also beapplied to a screw joint loosening operation, wherein the output torqueis temporarily reverse as one of the screw joints becomes fully loosenedso as to maintain a torque engagement with that screw joint.

We claim:
 1. A method for simultaneously tightening two or more screwjoints to a desired pretension condition by means of a nut running toolhaving two or more output spindles each having an adjustable outputtorque, said method comprising:individually operating each one of saidoutput spindles according to at least one primary tightening parameterfrom among torque, angular position, time, clampload, and screw jointelongation; continuously detecting an instantaneous value of said atleast one primary tightening parameter for each one of said outputspindles; continuously comparing said instantaneous tightening parametervalue for each one of said output spindles with a correspondinginstantaneous tightening parameter value of each other one of saidoutput spindles; and continuously adjusting a torque application speedby continuously adjusting a limit of the output torque of one of saidoutput spindles on which the instantaneous tightening parameter valueexceeds a predetermined boundary in relation to any other one of thespindles.
 2. The method according to claim 1, wherein the output torqueof each output spindle is controlled when the torque application speedbecomes zero.
 3. The method according to claim 2, wherein all of saidoutput spindles are stopped if the instantaneous tightening parametervalue of one of said output spindles differs by more than apredetermined amount from the instantaneous parameter value of anyoneother one of the output spindles.
 4. The method according to claim 3,wherein an amount of adjustment of said continuously adjusted outputtorque limit is a function of an amount of excess of said tighteningparameter value in relation to said predetermined boundary.
 5. Themethod according to claim 2, wherein an amount of adjustment of saidcontinuously adjusted output torque limit is a function of an amount ofexcess of said tightening parameter value in relation to saidpredetermined boundary.
 6. The method according to claim 1, wherein allof said output spindles are stopped if the instantaneous tighteningparameter value of one of said output spindles differs by more than apredetermined amount from the instantaneous parameter value of anyoneother one of the output spindles.
 7. The method according to claim 6,wherein an amount of adjustment of said continuously adjusted outputtorque limit is a function of an amount of excess of said tighteningparameter value in relation to said predetermined boundary.
 8. Themethod according to claim 1, wherein an amount of adjustment of saidcontinuously adjusted output torque limit is a function of an amount ofexcess of said tightening parameter value in relation to saidpredetermined boundary.
 9. A method for simultaneously tightening two ormore screw joints to a desired pretension condition by means of a nutrunning tool having two or more output spindles each having anadjustable output torque, said method comprising:continuously detectingfor each one of said output spindles an instantaneous value of at leastone primary tightening parameter from among torque, angular position,time, clampload, and screw joint elongation; selecting one of saidoutput spindles as a master spindle to be operated in accordance withsaid at least one primary tightening parameter; continuously comparingthe instantaneous tightening parameter value for said master spindlewith the instantaneous tightening parameter value of each other one ofsaid output spindles; and continuously adjusting a limit of the outputtorque of each other one of said output spindles so as to generate atightening parameter value that is substantially equal to theinstantaneous tightening parameter value of said master spindle.
 10. Themethod according to claim 9, wherein said at least one tighteningparameter is torque, said master spindle is operated to accomplish apredetermined torque growth characteristic as a function of time, andeach other one of said output spindles is individually controlled toadapt a torque growth characteristic thereof to conform with the torquegrowth characteristic accomplished by said master spindle.
 11. Themethod according to claim 10, wherein all of said output spindles arestopped if an instantaneous torque load on anyone of said outputspindles differs by more than a predetermined amount from aninstantaneous torque load on any other one of the output spindles.
 12. Amethod for loosening two or more screw joints by means of a nut runningtool having two or more output spindles each having an adjustable outputtorque, said method comprising:continuously detecting for each one ofsaid output spindles an instantaneous torque value; selecting one ofsaid output spindles as a master spindle to be operated in accordancewith said detected torque values; continuously comparing theinstantaneous torque value for said master spindle with theinstantaneous torque value of each other one of said output spindles;and continuously adjusting a limit of the output torque of each otherone of said output spindles so as to generate a torque value that issubstantially equal to the instantaneous torque value of said masterspindle; and wherein the output torque of a given one of said outputspindles which is engaged with a given one of said screw joints whichfirst becomes fully loosened is reversed so as to maintain a torqueengagement between said given one of said output spindles and said givenone of said screw joints which first becomes fully loosened.